Alkanoic acid esters by oxidation of alkyl-aromatic hydrocarbons with a group viii noble metal alkanoate



United States Patent() M US. Cl. 260488 11 Claims ABSTRACT OF THE DISCLOSURE A process for preparing carboxylic acid esters of a hydroxy alkyl aromatic Which consists of reacting an alkyl-aromatic hydrocarbon with a carboxylic acid salt of a Group VIII noble metal in the presence of a carboxylic acid.

This invention relates to a process for preparing a carboxylic acid ester of a hydroxy alkyl aromatic, wherein the hydroxyl group thereof is on the alkyl portion thereof, particularly to a process for preparing benzyl acetate.

I have found that carboxylic acid ester of a hydroxy alkyl aromatic can be prepared under relatively mild conditions by the expedient of heating an alkyl aromatic with a carboxylic acid salt of a noble metal.

The aromatic reactant employed herein can be a mononuclear, polynuclear or a substituted mononuclear or a substituted polynuclear aromatic hydrocarbon carrying an alkyl substituent thereon, branched or straight chain, having a carbon thereof directly. attached to a nuclear aromatic carbon, said attached carbon of said substituent carrying at least one hydrogen, preferably two hydrogens. The alkyl substituent can have from one to 40 carbon atoms, preferably from one to six carbon atoms, and can include methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, secondary butyl, isobutyl,- cyclobutyl, n-pentyl, cyclopentyl, 2-pentyl, 3-pentyl, 2-(3-methylbutyl), cyclohexyl, cyclopentylmethyl, 2-hexyl, 3-hexyl, 1-(2,3-di methylbutyl), n-hexyl, n-heptyl, 'n-tetr acontyl, n-pentadecyl, 4-eicosyl, n-(6-chlorohexyl), n (4-cyanotetradecyl),-, 7-(2-carbomethoxypentatriacontyl), n-decyl, etc. Examples of such alkyl aromatics that can be used include toluene, para xylene, meta xylene, ortho xylene, durene, ethylbenzene, para-diethylbenzene, cumene, a-cyclopropylnaphthalene, l-n-butylphenanthrene, para-secondarybutylbiphenyl, para-isobutyltoluene, cyclobutylferrocene, n-pentylbenzene, cyclopentylbenzene, l-methylbutylbenzene, cyclohexyl benzene, ot-cyclopentyltoluene, l-methylbutylbenzene, 2 (2,3 dimethylbutyl)phenanthrene, a-nhexylnaphthalene, n-heptylbenzene, n-pentadecylbenzene, 4-phenyleicosane, n-decylbenzene, 6-chlorohexylbenzene, 4-cyanotetradecylbenzene, 2-carbomethoxyl-7-phenylpentatriacontane, alpha-tetracontylnaphthalene, etc.

As noted the second reactant ernployed herein is a carboxylic acid salt of a noble metal. Thus, the cationic portion of the salt can be platinum, palladium, iridium, rhodium, osmium and ruthenium, preferably palladium, While the anionic portion thereof can be derived from the group of carboxylic acid salts, straight and branched chain, having from one to 40 carbon atoms, preferably from two to six carbon atoms. Examples of such carboxylic acids are formic, acetic, propionic, butyric, pivalic, octanoic, isooctanoic, benzoic, lauric, stearic, isobutyric, para-toluic, gamma-chlorobutyric, tetracontanoic, phenylacetic, cyclohexane carboxylic, crotonic, furoic, heptanoic, eicosanoic, etc. Examples of carboxylic acid salts of noble metals that can be employed include rhodium formate, palladium acetate, palladium propionate, iridium butyrate, palladium pivalate, palladium octanoate, osmium iso- 3,493,605 Patented Feb. 3, 1970 octanoate, palladium benzoate, palladium laurate, ruthenium stearate, palladium isobutyrate, palladium paratoluate, platinum gamma-chlorobutyrate, ruthenium tetracontanoate, osmium phenylacetate, iridium cyclohexane carboxylate, rhodium crotonate, palladium furoate, palladium heptanoate, palladium eiconsanoate, etc.

Preferably the aromatic compound and the carboxylic acid salt of the noble metal are heated in the presence of a solvent Which Will not adversely affect the course of the reaction and will not react to any appreciable extent with the reactants and/ or the products produced herein. Examples of such solvents are ethers, amides, sulfoxides, ketones, such as meta dioxane, dimethyl acetamide, dimethylformamide, dimethylsulfoxide, acetone, etc. and acids such as formic, acetic, propionic, butyric, isobutyric, valeric, hexanoic, heptanoic, gamma chlorobutyric, octanoic, methoxyacetic, etc. In a preferred embodiment, however, the solvent is a carboxylic acid corresponding to the anionic portion of the carboxylic acid salt of the noble metal employed in the present reaction.

The use of the specified carboxylic acid solvent is greatly advantageous in the present process. The use of noble metals in a chemical reaction is generally undesired, since the cost thereof is great, and their use can be justified, in many cases, if the amount of desired product obtained per unit amount of noble metal is large. The use of a carboxylic acid solvent enables the present process to have enhanced commercial possibilities, since it facilitates the reuse of the noble metal.

Thus, at the end of the reaction period, a reaction mixture is obtained containing the desired carboxylic acid ester of a hydroxy aromatic, the carboxylic acid solvent and the noble metal as a precipitate. The noble metal can be removed from the reaction mixture in any convenient manner, for example, by filtration. The ester, unreacted aromatic reactant, if present, and carboxylic acid solvent can also be separated from each other in any convenient manner, for example, by distillation. The noble metal and the carboxylic acid so recovered can then be brought together, heated and molecular oxygen passed therethrough to reoxidize the noble metal back to the noble metal salt for reuse in the reaction. The oxidation requires the use of the one mol of noble metal, tWo mols of carboxylic acid and one-half mol of molecular oxygen, resulting in the production of one mol of the noble metal salt and one mol of Water. The Water can be removed from the noble metal salt in any convenient manner, for example, by distillation. The oxidation procedure can involve a temperature of about 80 to about 180 C., an oxygen pressure of about 50 to about 3000 pounds per square inch gauge and a reaction time of about 0.1 to about 100 hours.

In heating the aromatic compound defined above with the noble metal salt of a carboxylic acid, I can employ the aromatic reactant and the noble metal salt in a molar ratio of about 1:1 to about 1000:1, preferably about 5:1 to about 100:1. Excess aromatic is desirable because it increases the rate of production of product, lessens the amount of overoxidation of the product and increases the efficiency of utilization of the noble metal salt. The

amount of solvent employed can be from about 0.1 to about 1006 mols, preferably from about one to about 50 mols, per mol of aromatic compound. The temperature employed during the reaction is desirably below the decomposition temperature of the noble metal salt. Thus, the temperature should not exceed about 200 C. and should preferably be below about 170 C. The temperature, however, should be above about 60 C., preferably above about C. The pressure is not critical and can range from about 0.1 to about 10,000 pounds per square inch gauge, preferably in the range of about 10 to about 1000 pounds per square inch gauge. The time of contact between the aromatic compound and the noble metal salt can be from about 0.1 to about 100 hours.

In order to increase the yields of desired ester and to render the process defined herein more specific thereto catalytic amounts of ionizable carboxylates, such as lithium acetate, sodium acetate, potassium acetate, rubidium acetate, caesium acetate, magnesium acetate, calcium propionate, barium acetate, strontium acetate, ammonium acetate, lithium butyrate, sodium benzoate, lanthanum cnotonoate, rubidium para-toluate, lithium pivalate, barium octanoate, potassium gamma-chlorobutyrate, aluminum propionate, etc. can also be added to the reaction mixture. In a preferred embodiment the carbOxylic acid forming the ionizable carboxylate corresponds to the anionic portion of the carboxylic acid salt of the noble metal employed in the present reaction. When employed, these compounds can be present in an amount that can range from about 0.1 to about 1000, preferably from about 5 to about 100, percent by weight based on the aromatic reactant.

The process of this invention can further be illustrated by the following.

EXAMPLE I TAB LE EtIiciency of benzyl acetate formation based on palladium acetate Lithium Palladium acetate, acetate, grams grams None None None Eflicieney in the last column= mols of benzyl acetate formed mols of palladium acetate charged It can be seen from the above data that an alkali acetate greatly improves the efiiciency of the present process. Additionally, a high ratio of aromatic compound to noble metal salt is desirable for good results.

EXAMPLE II Into a 50-milliliter reaction flask there was placed two millimols of palladium acetate, eight millimols of toluene, four millimols 0f chlorobenzene, a salt and enough acetic acid to form a total of milliliters of solution. The mixtures were refluxed at atmospheric pressure and 112 C. for hours. Chromatographic analysis of the product resulted in the following data.

Benzyl acetate yield based on palladium acetate,

Additional Component (percent) None Lithium acetate (five millimols) Cupric sulfate (five millimols) Cupric acetate (five millimols) Analysis of the product further showed that chlorobenzene failed to react. The above data further illustrate 4 that acetates of other metals improve the efliciency of the present process.

EXAMPLE III A mixture of 1.90 grams of toluene, 0.2410 gram of palladium acetate and 25 milliliters of propionic acid was refluxed at atmospheric pressure and C. for 20 hours. Analysis of the product by gas chromatography disclosed the presence of 5.5 millimols of benzyl propionate therein.

EXAMPLE IV A mixture of 20 millimols of para-xylene, one millimol of palladium acetate and sufficient acetic acid to form 25 milliliters of total solution was refluxed at atmospheric pressure and 1150 C. for 20 hours. Analysis of the product by gas chromatography disclosed the presence of 0.3 millimol of para-methylbenzyl acetate.

EXAMPLE V That the noble metal must be present as a carboxylic acid salt thereof is illustrated by the following. A mixture of one millimol of palladium metal, 20 millimols of toluene and suflicient acetic acid to form 25 millimols of total solution was refluxed for 20 hours. Gas chromatographic analysis showed no reaction had occurred. When this run was repeated with the same amount of palladium chloride in place of palladium metal, again no reaction occurred.

Obviously, many modifications and variations of the invention, as hereinabove set forth, can be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

Iclaim:

1. A process for preparing a carboxylic acid ester of a hydroxy alkyl aromatic, wherein the hydroxyl group thereof is on the alkyl portion thereof, which consists of heating at about 60200 C. an alkyl-aromatic hydrocarbon carrying an alkyl substituent thereon having a carbon thereof directly attached to a nuclear aromatic carbon, said attached carbon of said substituent carrying at least one hydrogen, with an alkanoic acid salt of a Group VIII noble metal in an alkanoic acid.

2. The process of claim 1 wherein said attached carbon of said substituent carries at least two hydrogens.

3. The process of claim 1 wherein said aromatic hydrocarbon reactant is toluene.

4. The process of claim 1 wherein said aromatic hydrocarbon reactant is xylene.

5. The process of claim 1 wherein said noble metal salt is a carboxylic acid salt of palladium.

6. The process of claim 1 wherein said noble metal salt is palladium acetate.

7. The process of claim 1 wherein said process is carried out in a carboxylic acid solvent corresponding to the anionic portion of said noble metal salt.

8. The process of claim 1 wherein said process is carried out in the acetic acid.

9. A process for preparing a carboxylic acid ester of a hydroxy alkyl aromatic, wherein the hydroxyl group thereof is on the alkyl portion thereof, which consists of heating at about 60-200 C. an alkyl-aromatic hydrocarbon carrying an alkyl substituent thereon having a carbon thereof directly attached to a nuclear aromatic carbon, said attached carbon of said substituent carrying at least one hydrogen, with an alkanoic acid salt of a Group VIII noble metal in an alkanoic acid wherein said reaction mixture additionally contains an ionizable alkanoate.

10. The process of claim 9 wherein said reaction mix-- ture additionally contains lithium acetate.

11. The process of claim 9 wherein said reaction mixture additionally contains cupric acetate.

(References on following page) References Cited LORRAINE A. WEINBERGER, Primary Examiner UNITED STATES PATENTS V. GARNER, Assistant Examiner 3,221,045 11/1965 McKeon et a1 260497 US. Cl- X'Rl FOREIGN PATENTS 5 260347.5, 410.5, 468, 476, 486, 487

981,987 2/1965 GreatBritain. 614,970 9/1962 Belgium. 

